Display apparatus having electroluminescence elements

ABSTRACT

A display apparatus includes display pixels each having a thin film transistor and an EL element formed successively forming over a substrate. The EL element has a cathode electrode connected to the source of the thin film transistor and an anode electrode, and is driven by the thin film transistor. The EL element externally emits light from the reverse side of the substrate. For example, when the cathode electrode is formed the comblike, meshlike, or gridlike pattern on the luminous layer, the light is emitted through the slits of the cathode pattern. The display apparatus is provided that can improve the aperture ratio of a display pixel and can increase the degree of freedom in deciding the size and the drive capability of a TFT element which drives an EL element.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 09/258,499, filed on Feb. 26, 1999 now U.S. Pat.No. 6,630,784, and which is herein incorporated by reference in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display apparatus that includeselectroluminescence (hereinafter referred to as EL) elements and thinfilm transistors (hereinafter referred to as TFTs) which are fabricatedon a substrate, and also to a method of fabricating the same.

2. Description of the Related Art

Recently, display apparatus using TFT elements and EL elements such asorganic EL elements have been noted as display devices, in place of theCRTs (Cathode Ray Tubes) or LCD (Liquid Crystal Display) apparatus.

FIG. 1 is a cross sectional view illustrating a conventional displayapparatus including organic EL elements and TFT elements. FIG. 1 shows alaminated structure in which an organic EL element is deposited over aTFT structure. The structure is formed as follows:

A gate electrode 2 is formed on a transparent insulating substrate 1such as glass or synthetic resin. An insulating film 3 is formed on thegate electrode 2. An active layer 4 of polycrystalline silicon is formedon the insulating film 3. A source region 4 s and a drain region 4 dinto which impurities are implanted are formed in the active layer 4. Aninterlayer insulating film 8 formed of a SiO₂ film 6 and a SiN film 7 isformed on the active layer 4. The source region 11 s is connected to thesource electrode 10 s via the contact hole 9 formed in the interlayerinsulating film 8. The drain region 4 d is connected to the drainelectrode 10 d via the contact hole 9 formed in the interlayerinsulating film 8.

Planarization insulating film 11 is formed on the electrodes 10 s and 10d and the interlayer insulating film 8. The source electrode 10 s isconnected to an anode electrode 28 of an organic EL element formed on aTFT element via the contact hole 12 formed in the planarizationinsulating film 11.

The organic EL element is formed by successively laminating an anode 2formed of: a transparent electrode of ITO (Indium Tin oxide); an organiclayer comprised a second hole transfer layer 27 of MTDATA(4,4′-bis(3-methylphenylphenylamino)biphenyl), a first hole transferlayer 26 of TPD (4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine),a luminous layer 25 of Bebq2(10-benzo[h]quinolinol-beryllium complex)containing Quinacridone derivative, and an electron transfer layer 24 ofBebq2; and a cathode electrode 23 of magnesium and indium alloy (MgIn).The organic layer is made of an organic chemical compound. Thus, an ELelement is constructed the organic layer, the anode electrode 28, andthe cathode electrode 23.

In the organic EL element, the holes injected from the anode electrode28 and the electrons injected from the cathode electrode 23 arerecombined inside the luminous layer 25. Excitons are generated byexciting the organic molecules of the luminous layer 25. The luminouslayer 25 radiates light through the process of the excitonsdisappearing. The light is radiated out from the transparent anodeelectrode 28 through the transparent insulating substrate 1.

However, in the conventional display apparatus structure, since theorganic EL element emits light through the side of the substrate 1 onwhich TFT elements are formed, the TFT structure blocks the emittedlight so that the display pixel aperture ratio cannot be increased.

Furthermore, since the TFT elements must be small-sized to the extentthat the luminous light is not blocked, there are severe limitations onincreasing the size of each TFT element as well as the TFT elementcapability.

SUMMARY OF THE INVENTION

The present invention is made to solve the above-mentioned problemsinvolved in the conventional display apparatus. It is an object of theinvention to provide a display apparatus that can improve the displaypixel aperture ratio and can increase the degree of freedom in decidingthe size and the drive capability of a thin film transistor which drivesan EL element.

According to the present invention, the display apparatus comprises: asubstrate; thin film transistors formed on the substrate, each of thethin film transistors having a source electrode and a drain electrode;and electroluminescence elements respectively formed over the thin filmtransistors, each of the electroluminescence elements having a cathodeelectrode, an anode electrode, and a luminous layer formed between thecathode electrode and the anode electrode; wherein each of theelectroluminescence elements emits toward the reverse side of thesubstrate.

Each of the electroluminescence elements comprises the cathodeelectrode, the luminous layer and the anode electrode successivelyformed above the thin film transistor. The cathode electrodes isconnected to a source or drain electrode of the corresponding thin filmtransistor. Moreover, each of the thin film transistors drives thecorresponding electroluminescence element.

Since light is emitted from the reverse side of the substrate, the thinfilm transistor formed on the substrate side does not block the light,so that the aperture ratio can be increased.

It is not required to miniaturize the thin film transistor to the extentthat the light is not shielded. Hence, the thin film transistor can bedesigned with high freedom of size. Thin film transistors with highperformance can be formed without constraints in size.

According to the present invention, the electroluminescence element isconstructed by successively forming a cathode electrode, a luminouslayer, and an anode electrode over the thin film transistor.

The anode electrode is made of a metal material and can cover over onlya part of the display pixel area within a unit display pixel area.

The above-mentioned planar structure can externally emit light from thereverse side of the substrate, that is, the anode side. Moreover, theanode electrode in, for example, a comblike, meshlike or gridlike formcan externally emit a sufficient amount of light.

The anode electrode of the electroluminescence element can be formed bya vapor evaporation process.

Furthermore, according to the present invention, the display apparatusis fabricated through the steps of forming the thin film transistors ona substrate; forming an insulating film to cover the thin filmtransistor; forming contact hole at predetermined position of theinsulating film, and then forming the cathode electrode of each of theelectroluminescence elements to respectively make contact with a sourceelectrode or a drain electrode of the thin film transistor via theholes; forming said luminous layer over the cathode electrode; andforming an anode electrode over the luminous layer using an opaque metalmaterial through a vapor evaporation method.The anode electrode is preferably formed to partially occupy a unitdisplay pixel region.

The electroluminescence element comprises an organic electroluminescenceelement using an organic material for the luminous layer.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other objects, features and advantages of the present inventionwill become more apparent upon a reading of the following detaileddescription and drawings, in which:

FIG. 1 is a cross sectional view schematically illustrating one displaypixel structure in a conventional color display apparatus;

FIG. 2 is a cross sectional view schematically illustrating one displaypixel structure in a fabrication step according to an embodiment of thepresent invention;

FIGS. 3A and 3B are cross sectional views each schematicallyillustrating the anode electrode for a display pixel according to anembodiment of the present invention; and

FIGS. 4A, 4B, 4C, 4D and 4E are cross sectional views each schematicallyillustrating a fabrication step according to an embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Next, an embodiment of a display apparatus according to the presentinvention will be described below with reference to the attacheddrawings.

FIG. 2 is a cross sectional view illustrating one pixel of a displayapparatus having organic EL elements and TFT elements, according to thepresent applicant.

The display apparatus shown in FIG. 2 differs from that shown in FIG. 1in that the TFT element and the organic element are reversely formed onthe substrate 1.

In each display pixel, an TFT element and an organic EL element arelaminated over an insulating substrate which is made of glass syntheticresin, or a conductive substrate or a semiconductor substrate on whichan insulating film such as SiO₂ film or SiN film is formed. Thesubstrate 1 may be a transparent or opaque substrate.

The TFT structure formed on the substrate 1 is the same as that of theconventional TFT structure, and so repeated explanation will be omittedhere. The source electrode 10 s is connected to the cathode electrode 13of an organic EL element formed over the TFT element via the contacthole formed in the planarization insulating film 11. The sourceelectrode 10 s supplies the drain signal from the TFT element to theorganic EL element via the drain signal line.

The organic EL element is formed by successively laminating a cathodeelectrode 13, an electron transfer layer 14, a luminous layer 15, firstand second hole transfer layers 16 and 17, and an anode electrode 18.The cathode electrode 13 comprises a magnesium and indium (MgIn) alloyor aluminum and lithium (AlLi) alloy and is connected to the sourceelectrode 10S of the TFT element. The electron transfer layer 14comprises of Bebq2. The luminous layer 15 comprises ofBebq2(10-benzo[h]quinolinol-beryllium complex) containing Quinacridonederivative. The first hole transfer layer 16 comprises TPD:triphenylamine dimer(4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine). the second holetransfer layer 17 comprises MTDATA(4,4′-bis(3-methylphenylphenylamino)biphenyl). The anode electrode 18comprises a metal electrode such as Pt, Rh or Pd and is shaped incomblike, meshlike or gridlike pattern (FIGS. 3A and 3B)).

In the luminous layer of each organic EL element, each of the electrontransfer layer, the luminous layer, the first hole transfer layer andthe second hole transfer layer comprises an organic chemical compound.Each organic layer is sandwiched between the anode electrode and thecathode electrode. The hole transfer layer may be a single layer.

The holes injected from the anode electrode 18 and the electronsinjected from the cathode electrode 13 are recombined together insidethe luminous layer 15. Organic molecules comprising the luminous layer15 are excited so that excitons are produced. The luminous layer 15emits light through the process of the excitons disappearing. Theemitted light is radiated out from the anode electrode 18 (to the uppervertical orientation in FIG. 2)

The light emitted from the luminous layer travels toward both thecathode electrode and the anode electrode but is reflected back from themetal cathode electrode without externally penetrating it. As a result,the light is radiated out through the slits of the anode electrode in acomblike, meshlike or gridlike pattern.

An organic EL display apparatus is constructed by arranging thethus-configured display pixels in a matrix form.

Here, the form of an anode electrode as well as the anode electrodefabricating method will be described below.

In the display apparatus of the invention having a laminated structurewhere an EL element is placed on a TFT element, the anode electrode isformed on the luminous element layer.

If the anode electrode 18 is formed on the luminous element layer by theITO (Indium Tin Oxide) sputtering process in the prior art, the luminouselement layer previously formed will be damaged.

According to the present invention, the anode electrode 18 is formed byvapor evaporating an opaque metal. This approach allows the anodeelectrode 18 to be formed over the luminous element layer with nooccurrence of damage.

However, if the anode electrode 18 is formed of a metal material overthe entire surface of a luminous element layer, the metal materialblocks the emitted light, so that the emitted light cannot be radiatedoutward. This means that the display apparatus does function normally.

In order to deal with such problems, the anode electrode 18 is formed ina comblike (FIG. 3A), meshlike or gridlike pattern (FIG. 3B), so thatlight is emitted from the reverse side of the substrate, that is,through the slits in the anode electrode 18 (in the arrow direction ofFIGS. 3A and 3B). The gap between comb teeth or the aperture size of themesh is selected to a brightness required as a display apparatus.

Next, the display apparatus fabricating method will be described below.FIGS. 4A to 4E are cross sectional views illustrating a process flow inmanufacturing a display apparatus according to the present invention.

In the step 1, as shown in FIG. 4A, a gate electrode 2 of a refractorymetal (a high-melting point metal) such as chromium (Cr) or molybdenum(Mo) is formed on the substrate 1 of which at least the surface isinsulative.

An insulating film 3 and a p-Si active layer 4 are formed all over thesurface of the substrate to cover the gate electrode 2. A stopper 5 ofSiO₂ film is formed on the p-Si film 4.

With the stopper 5 acting as a mask, P-type or N-type ions are dopedinto the p-Si film 4 to form the source region 4 s and the drain region4 d. The region masked by the stopper 5 and not doped with ions isdefined between the source region 4 s and the drain region 4 d and willact as a channel. An interlayer insulating film 8 formed of a SiO₂ film6 and a SiN film 7 is formed on the channel. A first contact hole 9penetrating the interlayer insulating film 8 is formed at the positioncorresponding to the source region 4 s while a first contact hole 9penetrating the interlayer insulating film 8 is formed at the positioncorresponding to the drain region 4 d. A source electrode 10 s is formedto connect to the source region 10 s via the first contact hole 9 whilea drain electrode 10 d is formed to connect to the drain region 10 d viathe first contact hole 9.

Thus, a TFT (poly-silicon thin film transistor, hereinafter referred toas “p-SiTFT”) which has a p-Si active layer and drives an organic ELelement is fabricated. The material for the active layer is not limitedto p-Si, but may be amorphous silicon or fine crystalline silicon.

Next, the step of forming an organic EL element on the TFT element willbe described below.

In the step 2, as shown in FIG. 4B, a planarization insulating film 11comprises on the insulating film 8 and the electrodes 10 s and 10 d ofthe p-Si TFT element. The planarization insulating film 11 comprises asilicon oxide film, a silicon nitride film, a silicon nitride oxidefilm, a silicate glass film, a SOG (Spin On Glass) film, or a syntheticresin film (ex. polyimide resin film, organic silica film or acrylicresin film). A contact hole 12 is formed in the planarization insulatingfilm 11.

In the step 3, as shown in FIG. 4C, the cathode electrode 13 of eithermagnesium and indium alloy (MgIn) or aluminum and lithium (AlLi) alloyof an organic EL element is formed on the planarization insulating film11. The cathode electrode 13 is connected to the source electrode 10 svia the contact hole 12 formed in the planarization insulating film 11.

In the step 4 shown in FIG. 4D, an electron transfer layer 14 comprisesBebq2, a luminous layer 15 comprisedBebq2(10-benzo[h]quinolinol-beryllium complex) containing quinacridonederivative, a first hole transfer layer 16 comprisedTPD(4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine), and a secondhole transf r layer 17 comprised MTDATA(4,4′-bis(3-methylphenylphenylamino)biphenyl) are successively over thecathode electrode 13.

In the step 5 shown in FIG. 4E, a metal film of, for example, Pt, Rh orRd is vapor evaporated on the second hole transfer layer 17 to form theanode electrode 18.

The anode electrode 18 may be formed by vacuum evaporating metal in anion state using the ion cluster method.

1. A method for manufacturing a display apparatus including a thin filmtransistor formed over a substrate, the thin film transistor having asource electrode and a drain electrode, and an electroluminescenceelement formed over the thin film transistor and having a luminouselement layer between a lower electrode and an upper electrode, whereinthe lower electrode, the luminous element layer, and the upper electrodeof the electroluminescence element are formed over the thin filmtransistor in that order, the upper electrode being formed using anon-transparent metal material and through evaporation so that the upperelectrode has a planar shape covering a portion of a display pixelregion in a unit display pixel region, and light emission from theelectroluminescence element is emitted to the outside from an upperelectrode side opposite of the substrate.
 2. A method for manufacturinga display apparatus according to claim 1, wherein one of the sourceelectrode and the drain electrode of the thin film transistor iselectrically connected to the lower electrode of the electroluminescenceelement.
 3. A method for manufacturing a display apparatus according toclaim 1, wherein light emission from the electroluminescence element isemitted through an opening region which is not covered by the upperelectrode within the unit display pixel region on the side near theupper electrode positioned opposite of the substrate.
 4. A method formanufacturing a display apparatus comprising the steps of: forming athin film transistor having a source electrode and a drain electrodeover a substrate; forming at least one layer of insulating film coveringthe thin film transistor; and after the formation of the insulatingfilm, forming an electroluminescence element having a luminous elementlayer between a lower electrode and an upper electrode, by forming alight reflective conductive material layer as a lower electrode, forminga luminous element layer, and forming an upper electrode on the luminouselement layer through evaporation of a non-transparent metal materiallayer having a planar shape covering a portion of a display pixel regionwithin a unit display pixel region, wherein light emission from theelectroluminescence element is emitted to the outside from the side nearthe upper electrode opposite of the substrate.
 5. A method formanufacturing a display apparatus according to claim 4, furthercomprising the steps of: forming a contact hole through the at least onelayer of insulating film covering the thin film transistor; andelectrically connecting one of the source electrode and the drainelectrode of the thin film transistor and the lower electrode of theelectroluminescence element.
 6. A method for manufacturing a displayapparatus according to claim 4, wherein light emission from theelectroluminescence element is emitted through an opening region whichis not covered by the upper electrode within the unit display pixelregion on the side near the upper electrode positioned opposite of thesubstrate.